This is the best emulator I have seen for Windows so far. It emulates just about every game image I have tried, bar one or two which could be bad dumps anyway.
This emulator is quite simple to use if you have owned an Atari 800 in youre life, and is pretty straight forward too if you have not. I'll give you a quick guide on getting a game up and running.

- Put the BIOS rom files into the BIOS folder
- Run the emulator, and setup how you want the graphics/input/sound etc. as you would with any emulator
- Load a Game Image by going to File->Attach Disk->Drive 1 then choose which game you would like.
- Cold Reset the Atari Machine by either hitting Shift+F5 or Atari->Cold Reset
- Jobs a good-un, and the rom should start to load. Speed up the emulation by pressing Button 3 on your controller (assuming your using a joystick/gamepad) so you dont have to wait through the loading. Press the button again to go to normal speed.
- If the game is asking you to press the START button, press F4

I chose this Emulator to be featured here because MESS has currently changed its Memory Core and this in turn has broke several drivers, including the good old Atari 800 so until it is fixed, I will concentrate on a working Emulator ;)

Leatherface